Typical Rankine Cycle power plants are steam power plants where water is used as working fluid or Organic Rankine Cycle “ORC” power plants where suitable organic vaporizable material such as for example one of the siloxanes is used as the working fluid. In many cases, there can be technical and/or economic issues which determine the most advantageous size of a Rankine Cycle power plant. For illustrative purposes, steam power plants are discussed below in more details. In many traditional steam power plants, a shaft of a steam turbine is sealed with contactless labyrinth seals, and steam flows through the labyrinth seals to the ambient air and prevents the ambient air from leaking to the steam system of the steam power plant. Therefore, there is a continuous steam-flow out from the steam power plant. As a corollary, there is a need for water feeding equipment which produces sufficiently purified feed water for substituting the water removed by the above-mentioned steam-flow. Furthermore, when the steam power plant is stopped and the above-mentioned steam-flow through the labyrinth seals is also stopped, the ambient air flows into the steam system of the steam power plant. Thus, a sufficiently efficient mechanism is needed for removing the air from the steam system when the steam power plant is started again. Furthermore, in many traditional steam power plants, the shafts of the steam turbine and the generator are supported with oil lubricated bearings which necessitate rotary oil seals and an oil circulations mechanism. Furthermore, especially in small steam power plants, there can be an oil lubricated gear between the steam turbine and the directly-network-connected generator. Due to the above-mentioned facts, a practical lower limit for the nominal power of a traditional steam power plant is about 3-5 MW because traditional steam power plants smaller than this are typically not cost effective.
A hermetic steam power plant comprises an electrical turbo-machine where an impeller or impellers of a steam turbine is/are directly connected to a rotor of a generator section of the electrical turbo-machine. Furthermore, bearings of the electrical turbo-machine are lubricated by water, an impeller of a supply pump is directly connected to the rotor of the generator section, and gas spaces of the generator section are filled with steam. In a hermetic steam power plant of the kind described above, there is no need for steam-flows through the labyrinth seals to the ambient air, no need for an oil circulations mechanism for the bearings of the electrical turbo-machine, and no need for a gearbox. Furthermore, the vacuum pump of a condenser of the hermetic steam power plant can be very small in comparison with that of a traditional steam power plant. In the hermetic steam power plant, the purpose of the vacuum pump is only to eliminate a possible air leak occurring through flanged seals. Hence, there is no need for such complex mechanisms as in conjunction with traditional steam power plants. Thus, a hermetic steam power plant can be cost effective also in cases where the nominal power is small. It is worth noting that the operating cycle of a hermetic power plant of the kind described above can also be based on vaporizable material other than water. For example, suitable organic vaporizable material such as for example one of the siloxanes can be used instead of water.
A hermetic power plant of the kind described above is, however, not free from challenges. One of the challenges is related to the cooling of the generator. Publication WO2008046957 describes a steam power plant where steam exiting from the turbine is circulated in the gas-space of the generator in order to cool the generator with the above-mentioned steam. The steam exiting the turbine can be, however, too hot for cooling the generator especially in cases where the steam power plant is a back-pressure power plant used for producing for example district heat. Using the hot steam for the cooling of the generator can be quite challenging because the hot steam may cause unwanted hydrolysis reactions which tend to damage typical electrically insulating materials. The damaging effect caused by the hydrolysis gets worse when the temperature rises.